2. Field of the Invention
The invention relates to a process for improving the color quality of moulding materials, which contain polyphenylene ether (PPE).
2. Discussion of the Background
It is well known that polyphenylene ether (PPE) is very sensitive to thermal stress. Even in the presence of oxygen, this sensitivity is evident in the formation of products that have a very dark color. Apparently these products are formed from the initial products, having quinonoid structures. These products are the by-products of the polycondensation of o,o-disubstituted phenols. Naturally contaminating by-products of all kinds effect to a considerable degree the color quality of PPE and PPE-containing moulding materials. This problem is very important if the PPE is not isolated by means of precipitation with a so-called anti-solvent but rather is worked up according to a so-called direct isolation process, which from the point of view of the process is more advantageous. In such a process, the catalyst components and by-products of the reaction such as diphenoquinone, which accumulate in the solution following the oxidative coupling reaction, are removed from the polymer; the solvent is removed to a large degree by distillation; and the remaining volatile components are removed from the material to a large degree in a extruder or a device which has a similar effect, and if necessary other polymer components, flame retardants, and other additives may be added.
Japanese patent application 70/40551 describes a process for stabilizing PPE with respect to thermal oxidation in which the OH end groups of PPE are reacted with styrene derivatives in the presence of basic compounds. This process has many disadvantages.
(a) Considerable quantities of monomeric styrene are required. After the PPE has been treated, the styrene derivatives that have not undergone reaction must be reisolated. PA1 (b) If the PPE were to be subjected to further treatment following the direct isolation process, the result would be a polymerization of the monomeric styrene. The result would be polymer mixtures, having a very low thermal dimensional stability due to their high styrene content. PA1 (c) The necessary addition of alkali compounds, p-benzoquinone, and piperazine is expensive.
JP-OS 71/02837 discloses that the end groups of the PPE can be reacted with acrylamides in the presence of basic compounds in order to shield the PPE from decomposition due to oxidation. Despite this fact, even in such masked polyphenylene ethers, thermal stress will result in unsatisfactory color changes. Thus one must assume that the observed reactions are not restricted to the end groups of PPE.
The addition of 0.1 to 6 percent by weight of organic phosphites as stabilizers is well known from the DE-PS 16 94 258. The effect of such stabilizers is that the PPE is not harmed by oxidation when the powder or the granulate undergoes further processing.
It has been proposed that chelating agents and other stabilizers such as, e.g., bivalent phenols and reducing agents (see DE-OS 27 54 887 and DE-PS 26 16 746) or aromatic amines (see DE-OS 27 55 937) be added. Whereas the chelating agents are supposed to alleviate the negative effects of residues of the metallic catalyst in the product, the object of the reducing agents is to convert colored quinones. These solutions are unsatisfactory, since the additives are often needed in large concentrations. In addition to this, it is also more difficult to isolate the polyphenylene ethers. In particular, with these techniques no color stability can be obtained at higher temperatures.
Another problem is that PPE alone and mixtures with syrene polymerizates tend to increase their molecular weight during the extrusion and the injection moulding process. This is unfavorable, since the flow properties of the polymer decline and during extrusion higher temperatures are required (see EP-OS 0 121 974, page 1). In order to obviate this, it has been proposed that a solid non-aromatic diene or dienophile be added to the polymer mixture.
According to DE-OS 24 30 130, the oxidative coupling reaction of monomeric phenols continues even after the actual reaction has been stopped. Thus on the one hand, a "zipper-like depolymerization reaction" occurs; and on the other hand, a cross-linking reaction occurs. The latter is the result of the hydrogen atoms at the alpha carbon atom splitting off from the side chain and active methylene groups being formed. These reactions also take place when the catalyst is inactive. Following the simultaneous treatment of PPE with bivalent phenol or benzoquinone and a mild reducing agent such as hydrazine, one observes that the molecular weight drops noticeably (see DE-PS 24 30 130). Thus in this manner it is possible to obtain a PPE, having a reproduceable molecular weight, following the oxidative coupling reaction. However, the problems that occur during extrusion are not solved in this fashion.
It is also known from the Japanese patent application 71/32427 that PPE can be stabilized by the addition of styrene in the presence of Bronsted acids.
Another process which is described in Japanese patent application 73/12197 discloses styrene polymerized in the presence of PPE and a radical initiator. In particular, the product is supposed to be heat resistant and to show resistance to chemical effects.
According to the data of the Japanese patent application 72/105414, graft copolymers, which are prepared in the presence of PPE and a radical initiator by means of polymerization of styrene, supposedly have an improved color quality. The last two processes have the drawback that they can be added only to mixtures that have a high content of polystyrenes.
The process of DE-OS 20 57 107 describes the grafting of styrene on PPE at a temperature ranging from 40.degree. to 150.degree. C. in the presence of a peroxide and water.